1. Field of the Invention
This invention relates to opposing hydraulic forces on the core, and control structure of the core, to maintain these internals of a nuclear reactor vessel in position. More particularly, the invention relates to clamping the barrel for core support, and the cylinder for support of the guide structure, between the upper internal ledge and the vessel closure to overcome hydraulic forces within the vessel which are directed toward displacing the core support and guide structure.
2. Description of the Prior Art
The nuclear reactor is structurally and functionally centered about its core, mounted in the lower portion of the reactor vessel. This core, comprised of fuel pins, is specificially supported within a cylinder arranged within the vessel as a liner and hung from a ledge formed at the top of the vessel. This core support barrel is perforated on its bottom so that liquid coolant can flow up through the core to be heated. Further, the core support barrel is penetrated above the core so that the heated liquid coolant can leave the vessel as a working fluid. The differential of hydraulic force of the coolant, across the core, is tremendous and is directed toward displacement of the core and its supporting structure.
The control rods of the core are reciprocated into and out of the core from above. These rods, and their reciprocating mechanism, are mounted in the plenum above the core where they are shrouded by tubes from direct contact with the coolant. As the heated coolant liquid passes through the plenum toward the vessel outlet, it exerts its tremendous force on these shroud tubes.
A second cylinder is telecoped within the core support barrel for support of the shroud tubes and mechanism associated with the control rods. This upper guide support structure is also hung from the upper vessel ledge and extends down to a short distance above the core. This structure is the basic subject matter of U.S. Pat. No. 3,849,257 issued Nov. 19, 1974 and opposes the hydraulic forces in the vessel plenum as they are developed on the shroud tubes.
In designing for the tremendous hydraulic forces generated on the in-vessel structure, the core support barrel and upper guide support structure have simply been flanged on their upper ends and thereby hung from the upper ledge of the reactor vessel. The heavy closure of the vessel has been sized to clamp the flanges of the barrels to the upper vessel ledge as the closure is bolted to the vessel top. If the differential expansion between the upper guide support barrel structure, core support barrel, vessel and closure were slight, the bolt up loading of the closure would provide a force large enough to prevent displacement of the barrels in opposition to the tremendous values of hydraulic forces imposed upon them. However, since the radial and axial differential expansion is large and must be accounted for to avoid both high stresses in the flanges and overloading of closure bolts, an alternative structure must be formed.
A flexible ring structure has been placed between the barrel flanges and the closure to transfer holddown force from the closure to the barrels. The effectiveness of a ring-type holddown device has been only marginally capable of providing the required holddown forces. As the size of reactor vessels and flow through them increases, the problem of generating the required holddown force through a ring-type device is more difficult because of size and space limitations. It is relatively easy to design a ring which has high load carrying potential. It is also easy to design a ring which can accept a large amount of deflection to accommodate differential expansion, mechanical tolerances and flange rotation due to bolt up and pressurization. It is very difficult, however, to achieve these combined characteristics with a single ring-type device.
Finally, there is the economic factor. The ring-type device is large and requires special, precise machining. It appears to be a reasonable goal to reduce these costs in excess of $50,000.
Spring assemblies have been placed between closures and barrel flanges hung on the vessel ledge. Specifically, stacks of Belleville springs have been placed in machined pockets of a ring between the closure and core support barrel flange to bear against the flange. This design crams a large number or relatively small-size Belleville springs in their specially machined ring pockets at the periphery of the flange on the upper guide structure. This structure is also expensive to fabricate because of the machining involved, the large amount of hand work and the inspection required.
A spring structure including the Belleville spring will solve the present problem. However, the spring structure must be mounted more simply and directly between the reactor vessel closure and both flanges of the upper guide structure and core support barrel in stabilizing the flanges on the vessel ledge.